This invention relates to a transfer device provided with a transfer roller, more specifically to a transfer device for electrostatically transferring a toner image from an image forming body to a copying medium.
In the field of electrostatic copying apparatus, for example, there have recently been proposed transfer devices in which a toner image is transferred to a sheet of copying paper as a copying medium by means of a transfer roller.
The transfer roller is composed of a sponge roller whose outer circumferential surface is covered with electrically conductive rubber or plastics impregnated with pulverized molybdenum sulfide, and an electrically conductive silicon tube covering the outer circumferential surface of the sponge roller. The transfer roller can be in rolling contact with the outer circumferential surface of an image forming body, such as e.g. a photosensitive drum. A bias potential is applied to the outer circumferential surface of the transfer roller. In a transfer operation, a sheet of copying paper is interposed between the contact surfaces of the photosensitive drum and the transfer roller so that a toner image formed on the photosensitive drum is electrostatically transferred to the copying paper. In cleaning the transfer roller after completion of the transfer process, moreover, a cleaning blade usually formed of rubber or plastics is held against the outer circumferential surface of the transfer roller to scrape off toner particles attached to the circumferential surface of the transfer roller.
There are two important requirements of the transfer device of this type; improved transfer efficiency and satisfactory cleaning effect.
In order to obtain the satisfactory cleaning effect, the surface of the transfer roller need be smooth and low in frictional resistance. Therefor, the surface of the transfer roller need be hard.
In order to improve the transfer efficiency, on the other hand, it is essential to increase the contact area of the transfer roller on the photosensitive drum. Therefore, the surface of the transfer roller need be soft.
Thus, with the aforementioned conventional construction of the transfer roller, it has been impossible to fulfill those two requirements at the same time.